Engine intake and exhaust valves are generally pushed in the closing direction by a spring, and are operated for example when a cam that rotates together with the engine pushes the valve in the reverse direction to the spring via a rocker arm.
The rocker arm is supported by a cylinder head such that it is free to oscillate via a rocker shaft. The cam is fixed on a cam shaft parallel to the rocker shaft. The cam shaft rotates together with the engine, and a rocker arm is made to pivot about the rocker shaft by a rotating cam that slides on the rocker arm.
A cam change-over mechanism that selects a plurality of cams in order to vary a lift amount and valve opening period according to engine running conditions is disclosed for example in U.S. Pat. No. 5,152,267. This mechanism comprises a main rocker arm that slides on a low speed cam and a sub-rocker arm that slides on a high speed cam. The operating characteristics of the valve are changed by linking and unlinking these rocker arms according to the engine running conditions.
In this cam change-over mechanism, the valve is always pushed by the main rocker arm, and the main rocker arm, when not engaged with the sub-rocker arm, is driven by the low speed cam. In this case the valve is operated according to the characteristics of the low speed cam. When a hydraulic piston housed in the main rocker arm penetrates the sub-rocker arm, the two rocker arms are engaged with each other and the valve is thereby operated according to the characteristics of the high speed cam.
The pressure that drives this hydraulic piston is supplied to the main rocker arm from a hydraulic passage formed in the center of the rocker shaft. Therefore, when the main rocker arm is displaced in the axial direction of the rocker shaft, an off port formed in the main rocker arm and another port in the rocker shaft are displaced relative to each other so that the flow of oil between the main rocker arm and rocker shaft is obstructed.
It is therefore necessary to restrict the displacement of the main rocker arm in the axial direction. This may be achieved for example by forming a hollow of effectively the same width as the main rocker arm in the cylinder head, installing the rocker shaft in the interior of this hollow, and supporting the main rocker arm by the rocker shaft.
This kind of hollow is generally formed by cutting the cylinder head with an end mill. The wall of the hollow in contact with the rocker arm specifies the position in the axial direction of the rocker arm, so this wall must be a flat vertical, accurately positioned surface.
However, as the wall is finished by pushing the side of the end mill against the wall surface so as to cut it, it is difficult to achieve this kind of precision, and the job is therefore time and labor consuming.